This invention relates to an electrical system comprising a power circuit and a control circuit wherein a PTC device is in one of the circuits and is thermally coupled to a resistive element in the other of said circuits. The PTC device protects the power circuit from excessive heat, current or both depending on the particular electrical system employed.
The term PTC device is used to describe an electrical device which undergoes a sharp increase in resistance, changing from a low resistance to a high resistance state, when the temperature of the device rises above a certain temperature. The use of certain PTC devices as circuit protection devices is disclosed in copending and commonly assigned application Ser. No. 965,344 of Middleman et. al., filed Dec. 1, 1978. The PTC devices of Ser. No. 965,344 are comprised of certain conductive polymer PTC compositions and possess specified characteristics which make them particularly useful in current protection applications. Use of other conductive polymer compositions in circuit protection devices has been proposed in the literature, for example in U.S. Pat. Nos. 2,978,665 (Vernet et. al) and 3,243,753 (Kohler). Ceramic PTC devices, for example, doped barium titanate devices, are also disclosed in the prior art to be capable of use as circuit protection devices (see for example, the article entitled "The PTC Resistor" by R. F. Blaha, in Proceedings of the Electronic Components Conference, 1971). Use of doped ceramic PTC devices in this manner is limited to low power circuits since doped ceramic devices typically have resistances greater than 2 ohms and cannot carry currents in excess of 500 milliamps. Use of a PTC device of either general type, as described in the prior art, contemplates use of the device in electrical series wih the load it is designed to protect. Excessive current through the circuit causes the device to heat by I.sup.2 R heating to above the temperature at which it changes to a high resistance state, thereby reducing current through the load. The device will also change to a high resistance state if the temperature of the device climbs above a certain temperature due to factors other than excessive current, eg. an increase in ambient temperature. The device stays in the high resistance state, effectively limiting current, until the device cools to temperatures at which it will revert to a low resistance state or, in the case of certain devices, until power is disconnected to the circuit and the device permitted to cool.
In certain electric apparatus it is not feasible or is impracticable to locate a current limiting device immediately adjacent the load where excessive heat might develop. In such cases a separate control circuit can be used. The control circuit usually includes a temperature-sensitive element, such as a thermistor, thermostat or thermocouple positioned next to the load, and means for limiting the current to the power circuit. Current limiting means that can be used include, for example, a thermal fuse, or an electrical or electronic switching device. In such control systems the current limiting means can be located remote from the power circuit to be protected as long as it is electrically connected therewith.